chicktwins, when you had them vaccinated for cocci when they were hatched, did you feed medicated feed? If you did, that negated the vaccine and it was like they were never vaccinated. Many people don't know that, so just checking. Cocci is really not that big of a deal, IF you know about the signs and watch for them and have Corid on hand to treat if need be. It's easily fixed if caught early on. Coccidiosis is very common, even with chicks who have had medicated feed (the med is amprolium, same as in Corid, but in a lesser dose), especially in wet climates like our soupy south.
I don't vaccinate chicks for anything here. I just watch for any signs and treat for 5 days with Corid, problem solved. Chicks being raised by broodies who live apart from the flock never need it.